This invention relates to molded case circuit breakers in general and more particularly relates to novel means for selectively increasing the interruption rating of one or more poles of a molded case circuit breaker.
Molded case circuit breakers often utilize a spaced plate type arc chute to receive and extinguish arcs generated during current interruption. The conductive plates of the arc chute serve to provide a series of barriers to the uniform outward expansion of an interruption arc in a direction away from the opening contacts. A serpentine-shaped arc results, as the arc may only expand into those areas between the spaced plates. This serpentine path causes rapid extinguishment as the arc travels into regions where the spaces between the plates of the arc chute become wider and the arc pressure is insufficient for the arc to bridge the increasing gap.
When a circuit breaker of the above described type is provided with bolted line terminal connections, it is desirable that the terminal mounting bolts be accessible from the front of the circuit breaker, so that the circuit breaker can be installed or removed from the front of a switchboard. Large current capacity breakers, in the range above 1000 amperes, utilize line terminals which require considerable mounting area. The volume above this area cannot be permanently filled with any other circuit breaker component if the terminal mounting bolts are to be accessible. The volume contained in front of these terminal areas is normally empty and wasted.
Most molded case circuit breakers are utilized at voltages which range between 120 volts and 480 volts. The remaining use of such molded case circuit breakers is generally at voltages in the region of 600 volts. As the voltage rises, the circuit breaker requires different and generally larger arc extinguishing structures.
One possible approach toward manufacturing a general purpose circuit breaker might be to design the breaker for use at 600 volts. When the breaker is used at voltages below 600 volts, a significant part of the arc structure is not required and its presence is economically and physically undesirable. Similarly, a circuit breaker designed to interrupt a particular maximum flow of current will have an increased current interrupting capacity from the addition of a larger arc chute in which de-ionization and proper quenching of the arc can take place. A circuit breaker designed to handle very large flows of current would also have a portion of its arc chute become unnecessary if subsequently used for lower current applications.
It is desirable to design and manufacture one basic circuit breaker which will handle moderate voltage and moderate current requirements, e.g. a voltage of 240V., a current of 10,000A., and then to provide for the installation of additional simple components to obtain a higher interruption rating. Existing compact circuit breaker contact structures, such as are described in my U.S. Pat. No. 3,770,992, and in U.S. Pat. No. 3,755,638, to Lucas and Huggins, will interrupt these wide ranges of voltages and currents. It has been found that the same type of contact structure may be utilized in a basic circuit breaker and obviate the necessity for replacement to increase the interruption rating somewhat. Similarly, it is known to provide a venting control, such as shown in my U.S. Pat. No. 3,803,376, to prevent flashover and to de-ionize arc products over an equally wide range of voltages and currents without requiring replacement to increase the interruption rating. However, an arc chute which is adequate at medium values becomes inadequate at higher values of voltage and current.
It is desired to selectively add a larger arc chute structure at the site of installation to meet higher voltage and current requirements than provided for by the basic circuit breaker, while still maintaining access to the line terminal mounting bolts. This larger arc chute would not be installed in circuit breakers whose voltage and current usage does not require the increased interruption ratings. This will result in a saving of material and manufacturing cost for the arc chute structure and will obviate the need to tool for several different interruption ratings for the same circuit breaker configuration.